Signal sources, sometimes referred to as frequency sources, are used to generate signals for use in many electronic systems. Applications requiring a source with both high-Q frequency stability/accuracy characteristics and frequency tunability generally require the use of a so-called “locked” signal source. In a locked source, a tunable source is locked to or otherwise derived from a fixed or “reference” source. Often the tunable source is locked to a reference source using one or more feedback circuits or feedback loops. In the case of a locked source using a feedback loop, once the feedback loop is closed the tunable source can achieve frequency stability and accuracy that are a function of the reference source frequency stability. Such a configuration of tunable and fixed sources locked together is known in the art as a synthesized source or simply as a frequency synthesizer.
A frequency synthesizer, then, is a signal source that generates an output signal from one or more reference signals. In general, a frequency synthesizer produces a signal including a single frequency selected from among a finite set of discrete frequencies available based upon the design of the synthesizer. Frequency synthesizers of various forms and designs are useful in a wide variety of applications including FM car radios, radar systems, handheld radios and test equipment, such as spectrum analyzers and signal generators.
In many cases, the synthesized signal produced by a given synthesizer is often at a higher frequency than that of the reference signal(s). The synthesized signal is typically a very stable, spectrally pure, single frequency signal having low or sometimes even very low phase noise. However, unlike other signal sources, such as a free-running voltage controlled oscillator (VCO), a given frequency synthesizer generally is capable of producing only a finite, although often large, number of selectable, discrete frequencies as an output signal. Therefore, frequency synthesizers are most often used where the frequency stability/precision and spectral purity are of paramount importance.
A number of different types of frequency synthesizers or methods of frequency synthesis are known including direct digital synthesis (DDS) and phase locked loop (PLL) frequency synthesis. The DDS uses a digital to analog converter (DAC) to convert a digital data stream into an analog output signal. The digital data stream is a digital representation of a sampled version of the desired output signal, thus the DDS directly synthesizes the output signal. In a PLL synthesizer, a feedback loop is used to compare and “phase lock” the output signal of a tunable frequency source, such as a VCO, to a stable reference signal produced by one or more reference sources. When locked, the PLL output frequency is typically a multiple of the reference signal or linear combination of the reference signal and other signals generated by the synthesizer. There are also hybrid synthesizers that combine one or more of these or other various frequency synthesis approaches.
A basic, single loop PLL synthesizer (SLS) used to synthesize a signal from a stable reference includes a voltage-controlled oscillator (VCO), a frequency divider, a reference oscillator, a phase/frequency comparator or detector (PFD), and a loop filter. The VCO produces an output signal, the frequency of which is proportional to an input control voltage. The frequency divider divides the output signal produced by the VCO to create a lower frequency signal. The frequency divider is a device that accepts a signal at a frequency f and produces an output signal at a frequency f/N where N is the division factor of the frequency divider.
The signal produced by the frequency divider is compared by the PFD to a reference frequency signal produced by the reference oscillator. The PFD, in turn, produces an error signal that is proportional to the phase/frequency difference between the frequency of the output signal of the frequency divider and the reference signal frequency, the error signal provided by the PFD output can take various forms, however commonly the error signal is presented as a “charge pump” configuration, where pulses of current form the error signal, The error signal is then filtered by the loop filter to produce the input control voltage of the VCO, the filtering is needed to remove the PFD comparison frequency and other spurious frequencies that would corrupt the spectral purity of the VCO, in the case of the “charge pump” PFD, the loop filter also integrates the current pulses to provide the DC control voltage that a VCO typically requires.
The action of the feedback loop of the PLL eventually causes or forces the error voltage to equal zero, a situation that is strictly true only when the output from frequency divider has the same phase as the reference signal. In essence, the VCO output signal is automatically adjusted by the feedback loop until the phase of the divided signal produced by the frequency divider equals the phase of the reference signal. Moreover, since the frequency of a signal is the derivative of the phase of the signal, for a pair of signals to have the same phase the signals must also have the same frequency. When the error voltage has been made equal to zero by the action of the feedback loop, the loop is said to be “locked” to the reference source.
Changing either the division factor N of the loop divider or the reference signal frequency can be used to change the frequency of the output signal. Generally but not always, the reference signal is fixed and the loop division factor is changed to affect tuning in a single loop frequency synthesizer (SLS).
Since low phase noise and minimizing spurious frequency content in the output signal are often key performance characteristics of a synthesizer, it is important to consider approaches that reduce or minimize the phase noise and spurious content. A synthesizer, with very high frequency operation and wide tuning range, typically requires special efforts to achieve good phase noise performance. Given that the phase noise “inside the loop bandwidth” (frequencies less than the loop filter frequency cutoff) in the single loop synthesizer is strongly effected by the value of N, one way to improve the phase noise “inside the loop bandwidth” is to reduce the division factor N. This can sometimes be accomplished in a satisfactory manner by increasing the reference signal frequency at the PFD.
However, while it is possible to increase the reference frequency to reduce N, there are limitations to this approach. In particular, unless a fractional-N loop divider (i.e. N not an integer) is used, the reference signal frequency at the PFD entirely determines the spacing between adjacent frequencies that can be synthesized by changing the N divider. The spacing between adjacent frequencies is often referred to as the “step size” or frequency resolution of the synthesizer. For example, if the reference signal frequency at the PFD is 10 MHz and N is an integer, the minimum step size of the synthesizer is 10 MHz. While fractional-N dividers can be used to lessen the step size limitations of using higher reference frequencies, there are additional problems with using fractional-N dividers, specifically the generation of fractional spurs which corrupt the spectral purity of the VCO.
A more commonly used technique to reduce N is to use an offset loop synthesizer. A basic offset loop frequency synthesizer (OLS) uses a low-noise, high frequency offset signal to downconvert or frequency shift the output signal of a main loop to a lower frequency prior to dividing the signal frequency in the main loop divider. The OLS comprises a VCO, an offset mixer, an offset signal source, a loop frequency divider, a PFD, a reference source, and a loop integrator.
The output signal of the VCO at a frequency f is mixed with an offset signal at a frequency fos. Generally, the offset signal frequency fos is chosen to be close to the output signal frequency f. The offset mixer, in turn, ideally produces output signals with the sum and the difference of the two input frequencies. The difference frequency is used to produce an intermediate frequency (IF) signal at a frequency fif. The difference signal is selected by using a lowpass filter and becomes the filtered IF signal. The filtered IF signal is then divided by the loop frequency divider and compared to the reference signal from the reference source by the PFD to produce an error voltage that is integrated by the loop integrator to produce the VCO control voltage. Typically, an output amplifier and a loop amplifier are used in the OLS, as mentioned above for the SLS.
When the loop is locked, the output signal frequency is equal to (fos−N*fref) or (fos+N*Fref), depending the polarity of the loop. Since the IF signal is at a frequency fif that is typically much lower than the VCO output signal frequency f, the division factor N required for a given reference signal frequency fref is typically much smaller than would be required for the SLS described above. Therefore, the phase noise gain associated with the loop division factor N is significantly reduced using an offset loop as in the OLS.
In sampler based synthesizers so-called crossing spurs may be a problem. In a synthesizer in which sampler IF signals cover a wide range of frequencies, nth order mixing products may fall inside the same frequency range as that of the sampler IF signal. These mixing products are generally unwanted and therefore spurious signals. Spurious signals resulting from nth order mixing products tend to move in frequency as the synthesizer is tuned, at a rate that is n times that of the desired, 1st order product. These spurs tend to cross the desired operational band, and are therefore referred to as crossing spurs. Unfortunately, since these nth order mixing products share the frequency range with that of the sampler IF signal, they cannot be filtered out using a filter.
While crossing spurs cannot typically be eliminated, the detrimental effect of crossing spurs can be reduced by decreasing the sampler IF signal frequency range. The narrower the frequency range of the sampler IF signal, the higher will be the order of the spurs that will cross the operational band of the sampler IF signal. Advantageously, the higher the order of a spur, the lower the amplitude of the spur and therefore, the less detrimental it is in terms of synthesizer performance.
Unfortunately, reducing the frequency range of the sampler IF signal for a given synthesizer design requires a larger selection of sample frequencies fs to minimize the sample frequency fs gaps. The frequency range of the sample frequency fs is generally determined by the minimum output frequency and the minimum sample frequency fs used. The maximum frequency gap that the sampler IF signal must cover is determined by the step size of sample frequency fs and the highest harmonic of the sample frequency fs being utilized. So there may be severe practical limitations on how narrow the sampler IF can be made in a given design.
Consequently, there are a number of conflicting requirements for the sampler-based PLL synthesizer. Compromises are often made to balance the requirements of the sample frequency fs and sampler IF signal range. The sampler output or sampler IF signal frequency range may need to be as small as possible to help to reduce the effect of crossing spurs and to simplify the requirements of the tuning reference or interpolation signal of the main loop. However, the frequency range of the sampler IF signal may need to be as large as possible to allow for a coarse stepped sample frequency fs. The coarser the frequency steps, the easier and less expensive it maybe to implement the sampling signal. The sampling signal should have a high frequency sampling signal frequency fs to minimize the harmonic number H. However, a small H requires the sample frequency fs to have a wider frequency range, making low phase noise more difficult to achieve. Moreover, the power level of the signal entering the sampler should be as high as is practical to produce the strongest sampler output signal and maximize the signal-to-noise ratio (SNR) of that signal. Maximizing the sampler output SNR helps to minimize the phase noise of the sampler. On the other hand, the signal level into the sampler should be as low as possible to minimize the generation of higher harmonics and thus to minimize power level of the crossing spurs.